Semiconductor memory devices are usually characterized as either volatile or non-volatile devices. In volatile memory devices, logic information is stored either by setting up the logic state of a bistable flip-flop, such as in a static random access memory, or through the charging of a capacitor as in a dynamic random access memory. In volatile memory devices, the data is lost whenever there is a power interruption; hence, the name volatile memories.
Non-volatile semiconductor memory devices such as mask read-only memory (MROM), programmable read-only memory (PROM), erasable read-only memory (EPROM), electrically erasable read-only memory (EEPROM), etc. are capable of storing data even during a power interruption. The nonvolatile memory data storage mode may be permanent or reprogrammable, depending upon the fabrication technology used. Nonvolatile memories may be used for program and microcode storage in a wide variety of applications in various industries, including the computer, avionics, telecommunications, and consumer electronics industries. A combination of single-chip volatile as well as nonvolatile memory storage modes is also available in devices such as nonvolatile SRAM (nvRAM) for systems that require fast, reprogrammable nonvolatile memories. In addition, numerous special memory architectures have evolved which contain some additional logic circuitry to optimize their performance for application-specific tasks.
In some nonvolatile semiconductor memory devices like MROM, PROM and EPROM, due to problems associated with electrical erasing and writing, it is not easy for general users to update the stored contents. On the other hand, since electrical erasing and writing for EEPROM can be readily accomplished, it is widely used in applications that need continuous updating. A flash EEPROM (hereinafter, referred to as a flash memory) has a higher integrity degree than a conventional EEPROM. Flash EEPROM is suitable for a large volume auxiliary storage device. Among flash memories, a NAND flash memory has a higher integrity degree as compared to a NOR flash memory. Generally, a NAND flash memory is used to store a large volume of data, and a NOR flash memory is used to store code data such as boot code.
With an increase in the use of flash memories, issues like operating time and power consumption of flash memories become critical design factors.